I. Field of the Invention
This invention relates generally to fire-fighting equipment, and more particularly to a system for accurately controlling the introduction of a liquid chemical foamant concentrate into a water stream so as to maintain a predetermined concentration of the foamant over a wide range of water flow rates.
II. Discussion of the Prior Art
In the Haugen et al. U.S. Pat. No. 5,174,383, assigned to assignee hereof, there is described an apparatus and method for controlling the introduction of a liquid chemical foamant concentrate into a water stream for enhancing the properties of the stream in fighting certain types of fires. In that system, water from a municipal supply or from a vehicle-carried tank is pumped through a hose and directed out of a nozzle unto the fire. The nozzle will typically have a variable orifice for controlling the flow rate. A flow meter is disposed in the water line supplying the hose and produces an electrical signal proportional to the rate of water flow through the hose. The chemical foamant concentrate is contained wit:bin a supply tank and a positive displacement piston pump having an adjustable piston stroke volume is arranged to be driven by a variable speed electric motor for pumping the foamant concentrate from the supply tank into the water stream. A microprocessor-based controller receives the electrical signal from the flow meter and another signal proportional to pump speed. It computes the rate at which the electric motor driving the pump must be driven to introduce a quantity of chemical foamant concentrate into the hose so as to maintain a pre-established percentage concentration of foamant in the water stream regardless of variations in water flow rates.
While the system described in the aforereferenced Haugen et al. Patent works well, it requires a special type of pump. The range of water flow rates over which the desired foam concentration can be maintained is somewhat limited.
The Arvidson et al. U.S. Pat. No. 5,232,052 entitled "APPARATUS AND METHOD FOR CONTROLLING THE INTRODUCTION OF CHEMICAL FOAMANT INTO A WATER STREAM IN FIRE-FIGHTING EQUIPMENT" describes a system which substantially increases the range over which the foam delivery pump can be operated in maintaining a relatively constant foam/water mixture concentration. In accordance with that invention, a DC motor is used to drive the pump and the speed of the motor is controlled over a first predetermined range using pulse width modulation of the DC motor drive current. The pump speed is a function of the duty cycle of the modulating waveform. It can be made to vary between a 100% duty cycle at which the motor is driven at its full rate of speed and about a 30% duty cycle at which the pump tends to stall. To achieve pump speeds below that which can be realized with the 30% duty cycle, the pulse width modulated signal may be further burst width modulated. Here, the pulse width modulated signal is turned on and off at predetermined time intervals determined by a microprocessor-based controller. Thus, the pump drive motor is intermittently driven in a stepped mode. In this fashion, the system is capable of injecting liquid chemical foamant measured in ounces per minute up to about 2.5 gallons per minute when the DC motor driving the pump is operating at its 100% duty cycle.
A need still exists for a system that can extend the range of foam concentrate delivery upward from what can be achieved using the system of the aforereferenced Arvidson et al. patent. For example, a municipal type fire truck used in fighting building fires will typically have a water cannon for delivering large volumes of foamed water onto the fire until it is substantially extinguished and then the fire-fighting personnel may have to use hand lines requiring relatively low flow for mop-up work. A water cannon may typically deliver 1000 gallons-per-minute while the hand lines involve water flow in the 50-100 gallon-per-minute range. Given that so-called Class A foams used in fighting building-type fires should be mixed with water so as to produce about a 1% solution of foam and water, the foam injection system must typically allow for foam concentrate injection rates from as low as 0.5 gallons per minute to as high as 10 gallons per minute.